Conventionally, many image processing techniques for performing position adjustment between multiple images such that specific target object images overlap have been proposed and used in various applications. These applications are typically comparison, sharpening and the like of images, but Patent Literature 1 (JP 2011-156321A) discloses yet another application. Patent Literature 1 discloses a technique for allowing a technician to accurately determine the relative positions of a wire and a blood vessel while viewing an X-ray video of a wire that has been inserted into the body of a patient. Specifically, a technique is disclosed in which position adjustment is performed between multiple frames included in the X-ray video such that the images of the wire overlap in order to remove the influence of motion in the wire images caused by the heartbeat, breathing, body movements, and the like of the patient. Also, in Patent Literature 1, as a first step for performing this kind of position adjustment, the wire images are automatically extracted from the frames included in the video.
If a wire in a body is to be photographed by X-ray, it may possibly be relatively easy to automatically extract the target object images from the frames. This is because in addition to having a unique shape inside a body, a wire absorbs more X-rays than the body of a patient that is captured in the background, and it is rendered relatively clearly. However, the accuracy of the automatic extraction of the target object images can greatly decrease depending on shooting conditions such as the type of target object and the shooting location. Furthermore, the accuracy can also decrease in the case where the shape or size of the target object image changes greatly during shooting. Accordingly, with the technique in Patent Literature 1, due to a decrease in the accuracy of the automatic extraction of the target object images, the accuracy of the position adjustment between frames included in the video may also decrease.